Single photon-phonon entanglement in WSe₂ quantum dots

We study photoluminescence of optically active quantum dots (QDs) in monolayer WSe₂ FET at low temperature (4K) and magnetic fields up to 8T. In addition to the neutral QD emission, a 21.8 meV red-shifted peak is observed. This replica peak shows correlated spectral jitters identical to the parent peak and exhibits similar excitation power dependence and magnetic field induced Zeeman splitting. Raman spectroscopy and phonon dispersion calculations are used to assign this peak to doubly-degenerate chiral phonons. Different to linearly polarized neutral QD, the phonon replica is completely unpolarized. This unusual behavior is not expected in a coherent phonon scattering. We understand it as a result of phonon-photon entanglement resulting from two indistinguishable paths in the phonon scattering process. As the phonon part of the entanglement is never measured, the photon part gets fully unpolarized. Under magnetic field this indistinguishability is lost and the replica peak recovers its polarization, corroborating our claim. Our results highlight an intriguing feature of 2D honeycomb lattice which can host chiral phonons. This single photon-phonon entanglement can be further explored for non-reciprocal light propagation at the quantum level.